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 Posted: 03-13-2005 10:11 pm
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Rory Clark
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In the final stage of 7 mo. rebuild. (73-JH #13218 49,480mile) Was able to put on road this week. it runs good with choke pulled 3/4 out and 20-BTDC @ idle, runs bad.  Backfires. With timing at 0-5 BTDC runs bad no matter choke in or out.  Checking the Vacuum there was no resistance suckin on the hose to Distr. so, pulled didstributor and it leaks air when pos or neg air pressure applied. And of course there is no movement in the advance.

Do I need to replace the complete Distributor or can I get just the Vacuum mechanism?   Any suggestions?

Rory Clark 13218

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 Posted: 03-14-2005 02:43 am
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Mark Rosenbaum
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Most JHs using Strombergs seem to need a fair amount of choke until they warm up.  If you need 3/4 choke for a warm engine, you may have carburetor problems.

I'm also a bit suspicious about your timing.  Any more or less stock JH should run quite well with a static timing between 8^ BTDC and 12^ BTDC.  They run less well (and much hotter) with timing in the 0^ BTDC - 5^ BTDC region, and quite poorly indeed when the timing is retarded beyond TDC.  If you have the wrong crankshaft pulley for your engine's front cover, the TDC mark will end up 10^ off (don't recall which way).  Unfortunately, determining actual TDC in comparison to the timing mark on the crank pulley can be a bit tedious, but may well be necessary if there's a real concern that you have the wrong part on the engine.

The vacuum retard mechanism was added to the JH distributor in an attempt to comply with emissions requirements at idle, but seems to have little actual effect.  Yours probably has a ruptured diaphagm, a common failing after 30-odd years.  Replacement vacuum capsules are unlikely to be available, repairs are all but impossible, and you can not safely use something from another British car unless it has very similar characteristics.

However, since the vacuum mechanism doesn't really do much, you can just ignore its failure and retain it only for the vernier timing adjust wheel.  Alternately, you could cut off the big disk for the vacuum mechanism and keep the barrel part, securing the spring and threaded shaft with epoxy.  Or you could throw it out and either fasten the two points plates together, or replace them with a single fixed one for a 23D series distributor (see Dave Bean's website. Usual disclaimers).

If you have to comply with emissions regulations that involve a visual check, leave the vacuum mechanism alone and in place, and plug the vacuum hose that runs to the distributor.  Otherwise, you can remove the vacuum line entirely and cap off the port on the tee to which it was connected -- or simplify things even further, and run a line directly from the port on the intake manifold, to the vacuum switch inlet.  If your engine's in even fair shape, and your carbs are fairly fresh, the car should still meet any state's emissions requirements.

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 Posted: 03-14-2005 12:46 pm
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Rory Clark
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Mark, Thanks for quick reply. Being in Oklahoma Emission control is a mute issue.

Just want to make sure. What you are saying. I can eliminate (plug) All vacuum line to the Carbs and Distrib. ??

I am a "Shade tree mechanic" from W-A-Y Back. this is my 6th rebuild of a British car But my dealings with twin Strombergs go back 20 years (TR-6) so I have forgot everything (Might be age too).

 Knowing that I can by-pass the Vacuum lines then I can get into the real problem. Carbs &/or pure timing. I have not rebuilt the carbs yet. (Have parts). I just want to make sure were to start. From what you said I need to start at the timing.

This week I will by-pass vacuum and set the static timing at 8 - 12 BTDC and then go from there. IF I have misunderstood about the vacuum please advise. 

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 Posted: 03-14-2005 04:36 pm
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Mark Rosenbaum
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In my younger days, shade trees were generally in short supply and much of my Brit car repairing was done in ditches, culverts, or the occasional grassy meadow.  I once had the fairly memorable experience of replacing a rear axle outer seal while being closely observed by a large and somewhat aggressive cow....

For troubleshooting on Stromberg-fed cars, all the vacuum lines can be capped, giving you a classical basic gasoline engine whose various systems have few if any unexpected interactions.

If the vacuum retard mechanism is not working, there's no point to running a vacuum line down to it, and the line can be removed.  This leaves an open port on a tee, which you'd need to cap.  Alternately, the tee and its lines can be removed and replaced with a single hose from intake manifold port to vacuum switch (the tower-like gadget added to the rear carb).

I do recommend leaving the vacuum switch and its associated plumbing in place and functional.  This is notionally an emissions system but it also aids in maintaining the correct thin layer of fuel on the inside walls of the intake runners.  If you do remove it, then neither the vacuum port in the center of the intake manifold, nor the throttle bypass valve on each carburetor, is necessary; the former can be plugged, the latter ignored or replaced with  blockoff plates.

You may also wish to leave the carbon cannister and its plumbing in place.  This is one of the few early emissions systems that not only (a) works more or less as advertised but (b) does not hinder performance.  However, the cannister and its lines can all be removed without harming anything.  In the event you do remove things, you'd need to cap the metal line running back to the fuel tank.  I would then also recommend replacing the spigot on the manifold flange of each carb with a small brass pipe plug, rather than using a rubber cap which will eventually crack and leak.  IIRC, these have a standard 1/8" pipe thread.

As a reminder, the brake booster is also vacuum operated, and failures there may cause apparent carburetion problems.

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 Posted: 03-14-2005 07:21 pm
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Rory Clark
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Totally understood and THANK YOU !!!!!!!!!!!!!!!

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